1. Field of the Invention
The present invention relates to a method and an apparatus for separating a desired gas component from a gas mixture containing a plurality of gas components.
2. Description of Related Art
Discharge into the atmosphere of hazardous gases including radioactive gases such as tritium (T), highly toxic gases such as dioxin, volatile gases such as hydrogen (H), or gases which accelerate global warming such as carbon dioxide, methane, or PFC (perfluoro compound) gases must be prevented. Therefore, gaseous mixtures containing these gases are processed in order to separate hazardous and benign gases.
Conventional techniques for gas separation will be described.
Compound Cryopump Method
Two plates which are cooled at a liquid helium temperature (xe2x88x92269xc2x0 C.) under super high vacuum of 10xe2x88x927xcx9c10xe2x88x928 Torr are prepared, and an adsorbent is applied to a surface of one of the two plates. When a gas mixture containing hydrogen isotopes including tritium, and helium is first brought into contact with the plate having no adsorbent applied, the hydrogen isotopes, which has a higher boiling point than helium, are condensed on this plate. Helium passes through this plate to reach the other plate bearing the adsorbent and is adsorbed on the adsorbent.
Because this method is followed under high vacuum, the volume of gases to be processed is enormous compared to a case where a gas is processed at around 1 atmosphere. This requires a significant increase in the surface area of a cooling plate on which a gas(es) is adsorbed, to thereby disadvantageously increase the size of a whole adsorption apparatus.
Pressure Swing Adsorption Method (hereinafter referred to as xe2x80x9cPSAxe2x80x9d)
The PSA method is widely used for separating various mixtures of gases. According to this method, for purification of hydrogen, for example, a column is first packed with a molecular sieve which functions as an adsorbent. A raw gas containing a mixture of hydrogen and gases having a molecular structure larger than that of hydrogen, such as carbon dioxide and methane gas, is fed to an adsorption column while the raw gas is pressurized to approximately 10xcx9c20 atmospheres, so as to have hydrogen adsorbed on the column while the remaining gases are discharged out of the column. Thereafter, the column is depressurized to the atmospheric pressure or a lower pressure, thereby desorbing hydrogen to obtain highly purified hydrogen. This method utilizes a change in the amount of a component to be adsorbed on the adsorbent according to the partial pressure of the component.
With this method, however, the adsorbing process should be performed under a high pressure in order to maximize the amount of a particular gas component that is adsorbed. For this reason, considerable effort must be exerted to ensure that any gas does not leak from the column and therefore this method is not suitable for treating any gas which becomes hazardous under high pressure.
Method Using Gas Separation Membrane
Hydrogen isotopes and helium have extremely high permeability to high polymer membranes and the like and can be separated from oxygen, nitrogen, hydrocarbons or the like according to a permeability difference therebetween. However, this method cannot separate gas components having substantially the same permeability, such as hydrogen isotopes and helium.
According to the present invention, at least three adsorption columns packed with an adsorbent are used. A raw gas containing a gas component A with low affinity with said adsorbent and a gas component C with high affinity with the adsorbent is sequentially fed to the adsorption columns in turn, while an desorption gas containing a gas component D which differs from the gas components A and C is fed to each of the adsorption columns other than the one to which the raw gas is being supplied.
When the raw gas is supplied to one of the adsorption columns, the gas component A in the raw gas having lower affinity with the adsorbent exits the adsorption column earlier than the gas component C in the raw gas having higher affinity. In this manner, the gas components A and C can be separated from each other.
According to another aspect of the present invention, when a gas enriched with the gas component A is discharged from the outlet of each adsorption column, all of the gas is extracted from the system. When a gas enriched with the gas component C is discharged from the outlet of each adsorption column, the full amount is extracted from the system. When the gas mixture containing the gas components A and C is discharged from the outlet of each adsorption column, all of the discharged gas mixture is fed back to the inlet of the adsorption column to which the raw gas is being supplied.
It is thus possible to efficiently and sequentially obtain the gas components A and C with a simple structure and efficient processing.
As described above, while the raw gas is supplied to the adsorption column, the gas components A and C can be collected separately from the adsorption column. This is because the component A having lower affinity with the adsorbent moves fast through the adsorption column while the component C having higher affinity with the adsorbent moves slow through the adsorption column, which causes a difference in time when the components A and C are released from the adsorption column.
The method according to the present invention can be applied under various additional conditions depending on specific gases to be separated. When separating a gas whose leak from the system is prohibited, a condition that such separation is performed with a pressure within the system being an atmosphere or less can be added.
In this case, gas discharge for each fraction can be achieved by a vacuum pump.
Further, an operation method which requires no pumps in the circulation line can be adopted by controlling the flow amount and pressure of the gas when supplying the gas mixture extracted from one adsorption column to another adsorption column.
Also, as a feature of the present invention, a concentration operation process can be performed. According to the concentration operation process, the component A or C is not extracted, but is accumulated within the system for some time. After the gas composition within the system is made significantly different from that of the raw gas, the gas is extracted out of the system.